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• Navigation Sensors
Filter Operation
The ECDIS includes an automatic multisensor Kalman filter for position. The filter can calculate the estimate of
position using simultaneously all operative position sensors.
Position sensors are shown by pressing SENSORS pushbutton.
Operator can manually include or exclude different sensors. Remaining sensors (= sensors with any other selection
than OFF) are included in the filter processing. The purpose of this filter is to continuously monitor the position
given by each individual sensor and define if the sensor is reliable or not. Also selected Gyro and Log sensor are
included into this analysis. The filter knows the different behaviour between different type of sensor. (Loran, Decca,
GPS, DGPS etc.). The result of the filter is a smooth position which is used to as final position of the ship. Filter
eliminates all jumps in the final position even if there is a position jumps in a sensor. (Decca line change, loss of
differential signal with DGPS etc.).
The filter can be selected or deselected by switching ON or OFF the status of the filter. If the filter is NOT selected,
the new start position of the estimated position is then the average value of the enabled sensors (sensors with any
other selection than OFF). If filter is selected, it operates from the alarm system point of view like a position source
- alarms are generated if the system do not have a valid position available. No alarms are generated for jumps or
validity of the position sensors selected to be used with filter as long as the filter is able to estimate a reliable
position based on selected position sensors.
Filter requires at least one position sensor and one course sensor (i.e. gyro) selected and valid in order to operate.
The filter has one main alarm "2000 Filter: Pos unreliable". This means that the filter is not able to calculate the
position from the connected sensors. This can happen, if there is only one position sensor connected and the position
of that one becomes unreliable or if there are more position sensors but they are deviating too much among them.
Alarm can appear also if the speed/ course sensor is found to be not reliable (Log or Gyro). Note that filter indicate
unreliable position sensors by changing the colour of their position into red.
If the filter is used without automatic Route Steering, then in case of alarm 2000 only filter itself is set in OFF state.
After the alarm 2000 the system will use its normal method to select position source from available sensor.
Normally this cause a jump in the position of the system. After the alarm 2000, the user should evaluate which
sensor caused the problem. Disable the bad sensor and then you can select the filter ON again.
Kalman filter is build to support automatic Route Steering and thus its model of the ship including navigational
sensors require that you have some speed. If the speed is less than 4 knots, the system generates alarm "2008 Filter:
Speed below 4 kn". If you don't use automatic Route Steering the system also selects the filter as OFF after the
alarm 2008. With automatic Route Steering the system keeps the filter selected, but repeats alarm 2008.
If there is a change in the availability of the sensor selected with Kalman Filter, the system generates alarm "2010
Filter: Pos source change". This change of available sensor may or may not affect the operation of the Filter.
However the user is always informed about a change.
Filter and automatic Route Steering
The filter can operate based on only one position sensor and one gyro. However this minimum selection is not
suitable for automatic Route Steering or program track turn, which both require an independent speed source. With
automatic Route Steering the minimum selection is for GOTO WAYPOINT:
• one position sensors, gyro and log or
• two position sensors and gyro
And for GOTO TRACK:
• one high precision position sensors, gyro and log or
• one high precision position sensor, second position sensors and gyro
Source of position indicates what kind of sensors the Filter is using for position calculation. The permanent
indication is available in upper right hand corner:
• FILTER, there is no high precision position sensor selected and available for Filter.
• FILTER HIGH, there is high precision position sensor selected and available for Filter. It is
possible to use track steering modes which requires accurate position (Goto Track, Program
Track Turn).
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